pISSN 1598-298X / eISSN 2384-0749 J Vet Clin 32(2) : 209-211 (2015) http://dx.doi.org/10.17555/jvc.2015.04.32.2.209 Hereditary Retinal Degeneration in a Siamese Cat Sang-wan Park, Seon-mi Kang, Eui-ri Lee, Soo-hyun Kim, Hyun-woo Noh, Ji-yoon Kwak and Kangmoon Seo1 Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Seoul National University, Seoul 151-742, Korea (Accepted: March 27, 2015) Abstract : A 5-year-old Siamese cat was presented at the Veterinary Medical Teaching Hospital in Seoul National University with visual impairment. The cat was blind and its pupillary light reflexes were sluggish OU. While there were no remarkable findings in the anterior segment, generalized tapetal hyperreflectivity and retinal vascular attenuation were observed in the posterior segment of both eyes. Any historical significance and clinical symptoms suggestive of infection or inflammation were absent. All of the above findings strongly indicated a diagnosis of hereditary retinal degeneration in the cat. This clinical case report demonstrates the possibility of the presence of feline hereditary retinal degeneration in Korea. Awareness of the condition should be raised to prevent the spread of hereditary retinopathy in the Korean cat population. Key words : blindness, cat, hereditary, retinal degeneration, Siamese. Introduction pressures (TonoVet® tonometer; Icare, Finland) measured 19 mmHg OD and 22 mmHg OS. A slit lamp biomicroscopic Most unsighted cats have a similar fundic appearance of examination (SL-D7, Topcon, Japan) showed no remarkable generalized retinal atrophy (3), because clinical detection of findings in the anterior segment. Indirect ophthalmoscopy the disease tends to take place later in cats than in dogs, was performed to evaluate the posterior segment. General- which makes it more difficult to determine the specific etiol- ized hyperreflectivity of the tapetal fundus was observed OU ogy (3). Clinicians usually attribute feline bilateral retinal and the retinal vasculature was severely attenuated OU. Fun- degeneration to one of following etiologies: nutrition, drugs, duscopic changes were documented using a digital fundus or hereditary retinopathy. camera (Fig 2). Nutritional retinopathy caused by taurine deficiency has The lack of historical significance and clinical symptoms become rare due to increased awareness of the necessity for suggestive of infection or inflammation, in combination with cat-specific food (4). An evaluation of cardiac function may the progressive nature of bilateral vision loss, most likely also aid in the diagnosis of taurine deficiency (4). In addi- indicates a case of hereditary retinal degeneration. Additional tion, careful history taking and a meticulous review of medi- examinations were refused by the client when she was cal records can establish the possibility of drug-induced informed that there is no specific treatment for retinal dis- retinal degeneration. Although genetic studies are needed to eases. confirm its hereditary basis, hereditary retinopathy can be diagnosed clinically through the exclusion of other etiologies. Discussion Case Feline retinal degeneration has been reported less fre- quently than canine cases, and the approach used by clinicians A five-year-old spayed female Siamese cat was presented for differential diagnosis should differ to the canine diagnos- at the Veterinary Medical Teaching Hospital in Seoul tic approach. National University with vision loss. The owner had noticed First of all, nutritional retinopathy caused by taurine defi- her visual impairment 6 months prior to the visit. There was ciency must be ruled out before hereditary retinopathy is no history of drug administration or anesthetic/surgical events, diagnosed. Recently, the incidence of nutritional retinopathy and the cat was fed only commercially available cat food. A has become rare due to an increased awareness of the neces- complete physical examination was unremarkable. But the sity for cat-specific food. However, cats living with dogs cat was blind OU, and incomplete and sluggish pupillary often eat dog food without their owners’ knowledge. Addi- light reflexes was observed OU (Fig 1). Tear production tionally, even though they have been fed cat-specific food, (Schirmer tear test®; Intervet, Summit, NJ, USA) measured taurine deficiency may develop if they have inherent amino 22 mm/min OD and 19 mm/min OS, and the intraocular acid absorption failure. In these cases, it is helpful to mea- sure plasma taurine concentrations in order to confirm taurine deficiency (4). Generally, however, taurine-deficient retinopa- 1Corresponding author. thy shows a characteristic oval hyperreflective lesion along E-mail : [email protected] the area centralis and is also accompanied by cardiomyopa- 209 210 Sang-wan Park, Seon-mi Kang, Eui-ri Lee, Soo-hyun Kim, Hyun-woo Noh, Ji-yoon Kwak and Kangmoon Seo Fig 1. Weak pupillary light reflexes were found in OU. A. The right eye. B. The left eye. Fig 2. Funduscopic photographs showed marked retinal atrophy, generalized tapetal hyperreflectivity and severe retinal vascular atten- uation OU. A. The right eye. B. The left eye. thy, which enables clinicians to differentiate it through care- rdAc allele. Studies show that rod degeneration precedes ful ophthalmic examination and cardiac function evaluation cone degeneration ultrastructurally by 4 to 5 months of age. (4,7,10). Clinically, however, the fundic appearance of homozygous Secondly, enrofloxacin associated retinal toxicity is one of (affected) cats is normal until 1.5 to 2 years of age. The end the possible etiologies of feline diffuse retinal degeneration. stage of blindness is usually reached by 3 to 7 years of age, One of the characteristics of the clinical course of this dis- implying that there is a great degree of phenotypic variation ease is the rapid progression of retinal degeneration, result- in rdAc disease under the influence of the uniform genetic ing in acute blindness (4,7,10). Research has proven that rod defect (5,6). degeneration precedes cone degeneration. Risk factor analy- Although most molecular genetic studies have focused on sis has shown that old age and hepatic or renal dysfunction Abyssinian cats, there are growing concerns about the pres- could reduce drug clearance, thereby increasing the risk of ence of hereditary retinopathy in Siamese cats (1,2). Breed fluoroquinolone retinal toxicity (4). predilection for the Siamese cat was observed in a retrospec- Thus, hereditary retinopathy can be considered when clini- tive study reported in 1999 (3). The mean age ± SD of the cians have ruled out nutritional, toxic, and inflammatory/ Siamese cats in this study was 12 ± 2.8 years, which is much infectious causes. Breeds identified to have a hereditary basis greater than the age of the patient described in this case. include the Abyssinian cat and the Somali cat (5). Of those Luckily, the owner in this case detected her cat’s visual breeds, many genetic and phenotypic studies have long been impairment earlier than most because she used to entertain performed on the Abyssinian cat (7,10). her cat with a laser pointer and realized that her cat had Two types of hereditary retinal dystrophy have been iden- become reluctant to look up at the moving laser beam. tified in Abyssinian cats. Early-onset retinal dystrophy, also Typically, cats’ exceptional ability to adapt to progressive known as cone-rod dystrophy (CoRD), is a primary photore- blindness makes their owners believe that they are visually ceptor disorder with an autosomal dominant mode of inherit- normal until their retina becomes completely atrophied (3). ance. Affected kittens show sluggish pupillary light reflexes In the initial stages of progressive retinal atrophy in dogs, and slightly dilated pupils by 4 to 5 weeks of age, leading to nyctalopia is commonly observed early by their owners (10). complete vision loss within the first 4 months of life, which This early clinical symptom, however, tends not to be noticed is in contrast to this case. Unlike CoRD, retinal degeneration by cat owners, making a timely diagnosis difficult in feline in Abyssinian cats (rdAc) is considered to be a late-onset dis- patients. Although the most reliable method for achieving an ease caused by autosomal recessive inheritance of the mutant early diagnosis of rdAc is to perform full-field flash elec- Hereditary Retinal Degeneration in a Siamese Cat 211 troretinograms (ERGs) at approximately 8 months of age (5), Institute for Veterinary Science(RIVS) of Seoul National most cats diagnosed late in the clinical course with complete Univesity, Korea. retinal atrophy are not suitable candidates for the evaluation of retinal function, making ERGs useless in general practice. References The prevalence of affected (homozygous) and carrier (het- erozygous) animals in the Abyssinian cat population was 1. Barnett KC. Retinal atrophy. Vet Rec 1965; 77: 1543-1552. found to be 45% and 44%, respectively (5). This unusually 2. Carlile JL. Feline retinal atrophy. Vet Rec 1981; 108: 311. high incidence of rdAc in Abyssinian cats is the result of fre- 3. Giuliano EA, van der Woerdt A. Feline Retinal Degeneration: quent close inbreeding among purebred Abyssinian cats. Clinical Experience and New Findings (1994-1997). J Am Recent reports have demonstrated that there is potential for Anim Hosp Assoc 1999; 35: 511-514. 4. Gelatt KN, Gilger BC, Kern TJ. Feline ophthalmology. In: the rdAc mutation to be present in breeds other than Abys- th Veterinary Ophthalmology, 5 ed. Ames: Wiley-Blackwell. sinian and Somali cats (5). One representative breed is the 2013: 1477-1559. Siamese cat, where rdAc allele frequency is approaching 5. Narfstrom K, David V, Jarret O, Beatty J, Barrs V, Wilkie 34% (6,9). The rdAc mutant allele has been known to be D, O'Brien S, Menotti-Raymond M. Retinal degeneration in transmitted to other popular cat breeds through crossbreed- the Abyssinian and Somali cat (rdAc): correlation between ing and the use of popular sires. The fact that rdAc is highly genotype and phenotype and rdAc allele frequency in two suspected in this Siamese case suggests that such genetic continents.